From the prior art there are known various methods for recognizing forged documents of value. For checking the authenticity of documents of value, in particular of bank notes, these can be checked as to their luminescence properties. The term luminescence is understood to be the generic term for the radiation which the document of value emits back upon optical excitation. This means that both fluorescence intensity and phosphorescence intensity can contribute to the luminescence intensity. For detecting the fluorescence signal and phosphorescence signal, these signals are detected e.g. timewise one after the other, the fluorescence signal during the illumination with an excitation light pulse, the phosphorescence signal after the end of the excitation light pulse in the dark phase between two excitation light poses. When using UV lamps, the light pulses of which do not end abruptly but have an afterglow, at the time the phosphorescence signal is measured there still remains a certain residuary intensity of the excitation light, which is due to the afterglow of the UV lamp. With this remaining excitation light fluorescent substances, which are present in a document of value, can be excited to form a residual fluorescence. The residual fluorescence emitted by the fluorescent substances contributes to a luminescence signal detected in the dark phase, so that the phosphorescence measurement in the dark phase is distorted.
For distinguishing authentic documents of value from forgeries e.g. there is checked whether the intensity of the phosphorescence signal of an examined document of value exceeds a certain threshold value, since many forgeries exclusively show fluorescence, but no phosphorescence. To recognize forgeries as such, the threshold value is set to be relatively high, so that the above-mentioned residual fluorescence of the forgeries cannot lead to an exceeding of the threshold value. But the high threshold value can lead to the fact that the phosphorescence signal of heavily soiled authentic documents of value does not achieve the threshold value either and these are inadvertently sorted out.